Photography is a popular pastime. Images, including digital images, can be created and manipulated using digital cameras, scanners, video equipment and image editing software. Images can be analyzed and modified in a variety of ways.
Images representing people often include substantial facial detail of the people represented. Facial detection involves determining whether images represent one or more faces. Facial detection also can involve determining in which portion of an image a face is represented, where particular features (e.g., eyes or a mouth) of the face are located, and the angle of the face relative to the overall image. Some facial detection processes focus on detection of frontal human faces. Other facial detection processes attempt to detect faces that are either rotated along the axis from the face to the observer (in-plane rotation), or rotated along the vertical or left-right axis (out-of-plane rotation), or both.
Image enhancement includes modifying characteristics of an image to potentially improve the appearance of the image. For example, an image can be enhanced by setting the black point of the image. The points of an image (e.g., pixels) can represent a range of colors. Setting the black point involves adjusting the range of colors so that at least some points are pure black. Similarly, setting a white point involves adjusting the colors of an image so that at least some points are pure white.
Image enhancement can result in clipping. Clipping includes modifying a range of colors such that some portion of the resulting range lies outside the limits of the medium. For example, pixel values of a grayscale image may be limited to values between 0 (pure black) and 255 (pure white) by an image format (e.g., the Portable Network Graphics (PNG) format). If one or more of an image's pixels have values of 255, further brightening of these pixels cannot be represented by the image format since the pixels are already at the maximum possible value. In addition, through the brightening operation image information can be lost as pixels with values near 255 are increased to 255 and become indistinguishable from the existing pixels with values of 255.
Images also can be enhanced by adjusting levels of saturation, contrast, color temperature and tint, exposure, definition, sharpness and other image characteristics. Saturation can refer to various concepts but generally refers to the intensity and purity of color relative to black, white or gray. Approximations of true color saturation can be employed as part of image enhancement.